(a) Field of the Invention
The present invention relates to a shift control method for an automatic transmission, and more particularly, to a shift control method for an automatic transmission that can reduce shift shock during 4-2 kickdown skip-shifting by learning-controlling a duty ratio of hydraulic fluid for controlling friction elements to be engaged or disengaged.
(b) Description of the Related Art
Shifting is generally performed in an automatic transmission in accordance with changes in throttle valve opening. That is, throttle valve opening is varied through driver manipulation of the accelerator pedal, and such changes in throttle valve opening correspond to different locations on a shift pattern such that shifting is performed.
There are instances when the driver may depress and release (or release then again depress) the accelerator pedal during a short interval of time. With the use of the conventional shift control method, shifting is automatically performed as dictated by the shift pattern. For example, if the driver releases the accelerator pedal during upshifting, the upshifting operation is completed, then shifting is directly performed back into the lower speed. This greatly reduces the quality of shift feel as shock is generated with such frequent shifting over a short time interval. Also, responsiveness deteriorates in the case where, for example, the accelerator pedal is again depressed during a downshift operation.
Particularly, during 4-2 kickdown skip-shifting control, since only hydraulic fluid directed to friction elements which will be engaged in a second speed is duty-controlled, it is impossible to correct hydraulic fluid when tie-up occurs. Accordingly, when over-run occurs by the shortage of hydraulic fluid of friction element to be disengaged, since only the friction element to be engaged is duty-controlled, shift feel deteriorates.
Furthermore, to duty-control hydraulic fluid directed to friction elements to be engaged without using a learning-control, a feed-back duty control which is performed in real time is required. However, it is time-consuming and costly to develop the feedback control.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a shift control method for an automatic transmission that reduces shift shock during the 4-2 kickdown skip-shifting by learning-controlling the duty ratio of hydraulic fluid directed to or released from friction elements to be engaged or disengaged.
To achieve the above objective, the present invention provides a shift control method for an automatic transmission comprising the steps of determining if an N-N-2 kickdown signal is input, determining if a real shifting time reaches a standard shifting time when the N-N-2 kickdown signal is input, determining if a turbine rpm variation is greater than a target turbine rpm variation at a shifting synchronizing point when the actual shifting time reaches the standard shifting time, determining if overrun occurs at friction elements to be engaged when the turbine rpm variation is greater than the target turbine rpm variation, and learning a hydraulic fluid control duty ratio by adding a predetermined value to a current duty ratio when the overrun occurs, then returning to a main routine.
The method may further comprise the steps of determining if overrun occurs at friction elements to be engaged when the turbine rpm variation is less than the target turbine rpm variation, and learning a hydraulic fluid control duty ratio by adding a predetermined value to the current duty ratio when the overrun occurs, then returning to a main routine.
The standard shifting time T is established according to a variation of a vehicle speed on the basis of data mapped by a pre-set program.
The shifting synchronizing point is established when the turbine RPM becomes larger than a value obtained by multiplying an output shaft RPM by the N speed gear ratio.
The method may further comprise the step of returning to the main routine without learning the hydraulic fluid control duty ratio when the overrun does not occur.
The method may further comprise the step of learning a hydraulic fluid control duty ratio by subtracting a predetermined value from a current duty ratio when the overrun does not occur, then returning to a main routine.